1. Field of the Invention
The present invention relates to a semiconductor device provided with an insulated gate element to be used for switching the main circuit of a power conversion apparatus such as an inverter apparatus or the like for electric motor control.
2. Description of the Related Art
Upon shorting of an output circuit of an inverter apparatus or the like using series semiconductor switching power elements, excessive electric power is applied to the switching power elements, thereby sometimes causing them to break down or fail. In order to improve the breakdown resistance of such power elements, conventionally, various contrivances have been made. As the conventional technique, there has been proposed, for example, "an insulated gate element driving circuit" as disclosed in Japanese Patent Application No. 63-129863 filed by the applicant of this application.
The insulated gate element is an element such as a power MOSFET driven to be on/off in response to presence or absence of a gate applied voltage. As the insulated gate element of the kind as described above, an IGBT is a typical one and therefore the insulated gate element is also called an IGBT hereinafter.
By way of contrast to the approach of the present invention, FIG. 4 shows an example of the circuit for driving an IGBT used as a switching power element disclosed in the foregoing Japanese Patent Application No. 63-129863. In the drawings, items the same as or equivalent to each other are referenced correspondingly. Further, the respective states of High and Low in logic value or level are simply represented by "H" and "L". In FIG. 4, the reference numeral 10 designates a DC power source (hereinafter, referred to as a gate driving power source), for example, of 15 V for driving the gate G of an IGBT 1, and the reference numerals 8 and 9 designate auxiliary transistors for interrupting the voltage of the gate driving source 10. The reference symbol e.sub.D designates a driving signal voltage for auxiliary transistors 8 and 9, and, thus, for the IGBT 1.
In the normal state, when the driving signal voltage e.sub.D is "H", the auxiliary transistors 8 and 9 are in their "off" and "on" states respectively, and the voltage of the gate driving power source 10 is applied across the gate G and emitter E of the IGBT 1 through auxiliary transistor 9 and the resistor 7 to thereby turn-on the IGBT 1, so that a main circuit current i.sub.o flows as a collector current between the collector C and emitter E of the IGBT 1 serially through a main circuit power source (not shown) and a main circuit load (not shown).
When the driving signal voltage e.sub.D is "L", on the contrary, the auxiliary transistors 8 and 9 are in their "on" and "off" states, respectively; and the gate driving power source is shut off from the gate circuit of the IGBT 1. At the same time, shorting occurs between the gate G and emitter E of the IGBT 1 through the resistor 7 and the auxiliary transistor 8 to thereby turn off the IGBT 1. Thus, the IGBT 1 is repeatedly turned on and off in accordance with the driving signal voltage e.sub.D, so that a current as required for the main circuit load flows.
Further, voltage dividing resistors 2 and 3 inserted between the collector C and emitter E of the IGBT 1 and a Zener diode 6 and an auxiliary transistor 5 inserted between the gate G and emitter E of the IGBT 1 are additionally provided for protecting the main circuit from being shorted.
That is, when the main circuit current i.sub.o has a normal value, the collector-emitter voltage eC.sub.E of the IGBT 1 is low, and the voltage across the voltage-dividing resistor 3 obtained by dividing the voltage eC.sub.E by the voltage-dividing resistors 2 and 3, that is, the base B--emitter E voltage eB.sub.E of the auxiliary transistor 5, is sufficiently low so that the auxiliary transistor 5 is kept "off" as it is. As a result, also the Zener diode 6 is kept non-conductive, and the gate G--emitter E voltage (hereinafter, referred to as a gate voltage) e.sub.g of the IGBT 1 is never influenced by the Zener diode 6 and the like, so that the IGBT 1 is driven by a sufficiently high gate voltage e.sub.g which is substantially equal to the voltage (about 15 V in this example) of the gate driving power source 10 and the collector-emitter voltage eC.sub.E of the IGBT 1 can become a sufficiently small value.
When the main circuit current i.sub.o has become excessively large because of shorting of the main serially-connected load circuit, on the contrary, the collector-emitter voltage ec.sub.E of the IGBT 1 becomes high, and therefore the base-emitter voltage eB.sub.E of the auxiliary transistor 5 becomes high to thereby turn-on the transistor 5 so that the gate voltage e.sub.g of the IGBT 1 is limited to the Zener voltage (about 7 V in this example) of the Zener diode 6. As a result, the overcurrent i.sub.o flowing in the main circuit is reduced in proportion to the reduction of the gate voltage e.sub.g of the IGBT 1, so that the time to the breakdown or failure of the IGBT 1 can be prolonged. Accordingly, it is possible to provide sufficient protection on account of the main circuit being shorted even by using a not-so-high-speed gate voltage turn-off circuit.
Nevertheless, the following problem requires solution.
In the foregoing circuit of FIG. 4, when the voltage between the main terminals of the switching power element rises upon shorting of an output circuit of an inverter apparatus in spite of the fact that a voltage is being applied to the control terminal thereof, the voltage across the control terminal is made to fall to thereby suppress the overcurrent. In such a method, however, it has been very difficult to integrate the circuits in one chip. This is because, particularly, the resistor 2 of FIG. 4 would then be integrated into the circuit; and it would then be difficult to raise the resistance value of the resistor 2 to a desirable value (to about 100K.OMEGA.). It is therefore an object of the present invention to solve the foregoing problem.